Multivibrator


Posted on Feb 7, 2014

A multivibrator is a form of relaxation oscillator which comprises two stages that are coupled so that the input of one is derived from the output of the other. A multivibrator is basically two amplifier circuits arranged with regenerative feedback. One of the amplifiers is conducting while the other is cut off. The Astablemultivibrator circuit has no stable state. With no external signal


Multivibrator
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applied, the transistors alternately switch from cutoff to saturation at a frequency determined by the RC time constants of the coupling circuits. So an Astable multivibrator could either be called a pulse generator or squarewave generator depending on the value of the resistor and capacitors used in the circuit. A related IC used is the 555 Timer configured as an 555 Astable multivibrator. The common configuration is to make each side of the circuit identical; The collector resistors R1 and R4 are equal to each other. The Base resistors R2 and R3 are equal to each other. The feedback capacitors C1 and C2 are equal to each other. As soon as power is applied one transistor is assumed to be in saturation while the other transistor is in cutoff. The time constants are defined by R2C1 and R3C2 which in turn set the high and low periods of the output pulse. The Monostable multivibrator circuit has one stable state; one transistor conducts while the other is cut off. The term One-Shot is also used to describe the circuit operation. An input signal [the Trigger] must be applied to change this condition. After a period of time, determined by the internal RC components [R2 and C1], a Monostable multivibrator circuit will return to its original condition where it remains until the next trigger signal arrives. This example uses two cross-coupled PNP transistors, with the pulse output derived from Q2. The length of the pulse is equal...




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